1. Field of the Invention
This invention relates to a catalyst for purifying diesel exhaust gas which purges a diesel engine exhaust gas of hydrocarbons (hereinafter referred to as “HC”), carbon monoxide (hereinafter referred to as “CO”), and soluble organic fractions (hereinafter referred to as “SOF”) contained therein by oxidation and decreases the discharge diesel particulates and a method for the production thereof.
2. Description of the Related Art
In recent years, the particulate matters particularly in the diesel engine exhaust gas (hereinafter referred to as “PM”) mainly comprise solid carbon particulates (hereinafter referred to as “SOOT”), sulfates (hereinafter referred to as “sulfates”), and liquid macromolecular hydrocarbon particulates (hereinafter referred to as “SOF”) and are posing a problem regarding environmental health. The reason for this anxiety resides in the fact that since the PM has a particle diameter of not more than 1 μm, it is liable to drift in the air and enter the human body through breathing. Thus, the authorities are formulating a policy toward exacting the control imposed on the emission of the PM from the diesel engines.
Meantime, the amount of the PM emitted from the diesel engine has been appreciably diminished owing to the optimization of the operation of the diesel engine materialized by the electronic control of the fuel injection system directed toward exalting its pressure and adapting its timing. This diminution, however, hardly deserves to be rated satisfactory. Besides, the SOF in the PM contains such harmful fractions as cancerating substances. Thus, the elimination of SOF and SOOT as well has come to form an important problem.
In search of a means to remove the PM, the feasibility of a system of catalysis which seizes the PM fraction in the diesel engine exhaust gas by using a device formed by depositing a catalytic substance capable of burning the PM on such a refractory three-dimensional structure as a clogged type ceramic honeycomb, a ceramic foam, a wire mesh, a metallic foam, an open-flow type ceramic honeycomb, or a metallic honeycomb and as well removes the PM either by forcibly elevating the temperature of the engine under the exhaust gas emission conditions (gas composition and temperature) obtained under the ordinary traveling condition of the diesel engine or through control of the engine or by using such a heating means as an electric heater has been being studied.
Generally, as the catalyst for purifying the exhaust gas of the diesel engine particularly of the grade intended for use in the passenger car, (1) a catalyst manifesting high efficiency in the removal by combustion of such harmful fractions as unburned hydrocarbons (hereinafter referred to as “HC”) and carbon monoxide (hereinafter referred to as “CO”) besides PM, (2) a catalyst excelling in the so-called high-temperature stability, namely the ability to endure a continuous operation under a high load, and (3) a catalyst excelling in the so-called ability to resist sulfur poisoning, namely the insusceptibility to the poisoning caused by the adhesion of a sulfur oxide on the catalyst observed in the operation under a low load at a low emission gas temperature are being sought.
Various proposals have been hitherto advanced with the object of exalting the efficiency of the removal by combustion of PM, HI, and CO in the diesel engine exhaust gas. The official gazette of JP-A-HEI 7-24260, for example, discloses a catalyst having platinum and/or palladium and the oxide of at least one element selected from the group consisting of tungsten, antimony, molybdenum, nickel, vanadium, manganese, iron, bismuth, cobalt, zinc, and alkaline earth metals deposited on a refractory inorganic oxide in an amount in the range of 5-50 mass % based on the mass of the inorganic oxide.
Then, in the official gazette of JP-A-HEI 10-33986, a catalyst which possesses several kinds of zeolite mixtures having different moduli and a platinum family metal and a separate metal oxide selected from the group consisting of aluminum silicate, aluminum oxide, and titanium oxide, has the aluminum silicate contain silicate dioxide at a weight ratio in the range of 0.005-1 relative to aluminum oxide, and has the platinum family metal deposited exclusively on the separate metal oxid is disclosed.
The temperature of the engine exhaust gas tends to fall under the influence such as of the EGR which is mounted on the engine for the purpose of decreasing the NOx in the engine exhaust gas. The change of the control test mode to the cold start condition with a view to approximating it to the actual working environment of the car is also spurring the decrease of the exhaust gas temperature. Under these situations, the conventional catalysts mentioned above are still deficient in the ability invariably in the low-temperature regions of PM, HC, and CO, though they are appreciably capable of combustion. During the operation at a low exhaust gas temperature, they are susceptible of the poisoning caused by the deposition of sulfur oxide on the catalyst and therefore are at a disadvantage in suffering the efficiency of combustion of PM, HC, and CO to decline.
This invention has been initiated in appreciation of this true status of affairs and has an object thereof the creation of a catalyst of the quality defined in (1)-(3) mentioned above for the purification of a diesel engine exhaust gas, which catalyst possesses satisfactorily the properties particularly required of a catalyst for use on the passenger car.